(1) Field of the Invention
The present invention relates to an image forming apparatus and a developing apparatus for developing a latent image formed on a photoreceptor drum, using a developer containing a toner and a carrier.
(2) Description of the Related Art
There are developing apparatuses, among those for use in image forming apparatuses, that adopt what is called two-component developing system. According to the developing system, a mixture of toner particles and carrier particles is stirred to generate electrical charge by the friction between them. The carrier is held on the surface of the development sleeve while the toner is attached to the carrier such that the toner is transferred via the carrier to the latent image on the photoreceptor drum so as to develop the latent image. In the two-component developing system, the development performance is influenced by the charge characteristics of the toner and the carrier to a great degree. It is accordingly required that the charge characteristics of the toner and the carrier are kept to be in good condition all through the life of the developer.
Japanese Laid-Open Patent Application No. 10-26885, for example, discloses an image forming apparatus that, if images requiring a small amount of developer are developed in succession, separates the two-component developer into the toner and the carrier to prevent the toner charge-up (excessive charge) or degradation of the carrier.
Meanwhile, in the two-component developing system, while the developer is in use, finely-divided toner particles are generated due to cuts and cracks of the toner particles that are caused by the friction between the carrier and toner.
The finely-divided toner particles are apt to attach to the surface of the carrier particles, and are gradually accumulated thereon. The more amount of finely-divided toner particles the carrier surface accumulates, the less amount of electrical charge the friction between the toner and carrier particles generates, and the less amount of electrical charge the toner has.
Japanese Laid-Open Patent Application No. 10-26885 discloses the technology for separating the toner and the carrier, but fails to consider the finely-divided toner particles. For this reason, the reduction in toner charge is observed also in the image forming apparatus of the above-mentioned Japanese patent application.
The reduction in toner charge leads to the degradation in image quality. A carrier that does not have finely-divided toner particles on the surface thereof can be obtained by changing the developer. However, frequently changing the developer increases the time and effort of the maintenance and the cost. It is therefore desirable that the developer has as long a life as possible.